The invention described herein relates to nuclear reactor fuel assemblies and more particularly to a zircaloy fuel assembly grid designed to improve strength, and reactor performance, and to be manufactured at a cost less than conventional grids.
It is well known that the fuel or fissionable material for heterogeneous nuclear reactors is conventionally in the form of fuel elements or rods which are grouped together. These groupings or fuel assemblies also include rods comprising burnable poisons and hollow tubes through which control element assemblies are arranged to pass. The liquid moderator-coolant, normally water, flows upwardly through the reactor core in channels or longitudinal passageways formed between the members that comprise the core. One of the operating limitations on current reactors is established by the onset of film boiling on the surfaces of the fuel elements. The phenomenon is commonly referred to as departure from nucleate boiling (DNB) and is affected by the fuel element spacing, system pressure, heat flux, coolant enthalpy and coolant velocity. When DNB occurs, there is a rapid rise in the temperature of the fuel element due to the reduced heat transfer which can ultimately result in failure of the element. Therefore, in order to maintain a factor of safety, nuclear reactors must be operated at a heat flux level somewhat lower than that at which DNB occurs. This margin is commonly referred to as the "thermal margin".
Nuclear reactors normally have some regions in the core which have a higher neutron flux and power density than other regions. This situation may be caused by a number of factors, one of which is the presence of control rod channels in the core. When the control rods are withdrawn, these channels are filled with moderator which increases the local moderating capacity and thereby increases the power generated in the fuel. In these regions of high power density known as "hot channels", there is a higher rate of enthalpy rise than in other channels. It is such hot channels that set the maximum operating conditions for the reactor and limit the amount of power that can be generated, since it is in these channels that the critical thermal margin is first reached.
Attempts have been made in the past to solve these problems and increase DNB performance by providing the support grid structures employed to contain the members of the fuel assembly with integral flow deflector vanes. These vanes can improve performance by increasing coolant mixing and rod heat transfer ability downstream of the vanes. These attempts to improve performance have met with varying success depending on the vane design and the design of other grid components which can impact the effectiveness of vanes. To maximize the benefit of the vanes, the size, shape, bend angle, and location of the vanes must be optimized. The vanes are especially beneficial adjacent to the aforementioned hot channels. The remaining components of the grid which include the strips, rod support features and welds must be streamlined to reduce the turbulence generated in the vicinity of the vanes. Further constraints on designing the grids include minimizing grid pressure drop and maximizing grid load carrying strength.
Grids are generally of a first and second plurality of half-slotted strips in "egg-crate" configuration and are spaced along the fuel assembly to provide support for the fuel rods, maintain fuel rod spacing, promote mixing of coolant, provide lateral support and positioning for control assembly guide tubes, and provide lateral support and positioning for an instrumentation tube. The grid assembly usually consists of individual strips that interlock to form a lattice. The resulting square cells provide support for the fuel rods in two perpendicular planes; in general, each plane has three support points: two support arches and one spring. The springs and arches are stamped and formed in the grid strip and thus are integral parts of the grid assembly. The springs exert a controlled force, preset so as to optimally maintain the spring force on the fuel rod over the operating life of the fuel assembly.
Fuel assemblies employing spacer grids with flow deflector vanes of the prior art have usually been fabricated substantially or entirely of Inconel or a zirconium-tin alloy, i.e., zircaloy. An Inconel grid has the advantage of greater strength because of better material characteristics and because the brazing process bonds the intersection of the strips along its entire length. Brazing also has the advantage of providing little or no obstruction to flow. Due to the increased strength, the strip thickness of an Inconel grid can be reduced relative to the zircaloy grid to reduce pressure drop and turbulence in the vicinity of the vanes. The use of annealed zircaloy has been directed by its desirable combination of mechanical strength, workability, and low neutron capture cross-section. The most important of these characteristics is its low neutron capture cross-section which makes the nuclear fission more efficient, thus making the nuclear reactor operate more economically. However, to achieve a strength equivalent to that of an Inconel grid, the strip thickness for a zircaloy grid must be increased, thus creating more turbulence and higher pressure drop. Also, the joining of the interlocking zircaloy strips has always been by welding which requires the melting of some grid material to form a weld nugget. The increases strip thickness and weld nuggets for zircaloy grids of the prior art increase turbulence and grid pressure drop and reduce the effectiveness of the vanes. Therefore, the DNB performance of a zircaloy grid containing flow vanes of the prior art will be degraded relative to an Inconel grid design.
In U.S. Pat. No. 4,089,741, a split vaned grid is disclosed in which first and second welding tabs are disposed in intersecting relation. Fusing of the protruding tabs at the intersection points down into the intersection joints occurs such that the protruding tabs are consumed whereupon there is formed in said vanes an opening at the base thereof, but within the bent and flow exposed vanes and not the vertical sections supporting them. The openings have a shape of the same general configuration as that of said first protruding tab, whereby flow is through the opening and in that patent, it is alleged the flow mixing capability of said spacer is improved.
FIG. 1 is a prior art view showing what happens to create flow separation when a vane such as that of U.S. Pat. No. 4,089,741 has a nugget weld "unshielded" from the flow and an opening in the vane itself.
U.S. Pat. Application Ser. No. 856,888 of Donald W. Krawiec, now U.S. Pat. No. 4,725,402 assigned to the assignee of the instant invention teaches "shielding" the weld nugget from the flow path within the confines of the strips in openings along their lines of intersection, to minimize pressure drop. This application does not specifically disclose integral vanes of the type in U.S. Pat. No. 4,089,741, however, which have openings which increase pressure drop by flow separation during flow therethrough.
Water table tests were performed to visualize how the weld nugget and the welding hole cutout in the prior art vane for a nugget affects the flow passing by and through the vane. FIG. 1 illustrates the prior art flow pattern with a nugget and its weld hole in the vane. It can be seen that the weld nugget/weld access hole generates a very large wake, which, in turn, promotes decay of the vane effectiveness downstream of the grid. Velocity measurements downstream of the grid, both in water table tests and in an air model, using Laser-Doppler Anemometry, support the claim that the vane of the invention is more effective in directing the flow into the fuel rod gap because the weld nugget/weld access hole is not present in the vane.